WebServer: if ipv6 is available, try to start two MHD instances,
one on ipv6, then one on ipv4. On dual-stack systems where listening
on ipv6 also accepts ipv4 connections (at least OSX), only
one thread will be used and serve both address families.
On others, two threads will be started.
Merge branch 'master' of github.iserv.nl:simonvetter/xbmc